Fluid level sensing

ABSTRACT

A tank to contain a fluid includes a tank wall having an inner surface. A vane is disposed within the tank. The vane is configured to facilitate extraction of fluid from the tank. The vane may include an electrically conductive material. The tank also includes a first connector to electrically couple at least one portion of the tank wall to a capacitance sensing device and a second connector to electrically couple at least one portion of the vane to the capacitance sensing device.

FIELD OF THE DISCLOSURE

The present disclosure is generally related to fluid level sensing.

BACKGROUND

Accurate determination of a quantity of a fluid inside a tank can bedifficult. For example, certain methods of estimating an amount of aliquid propellant that remains in a satellite's fuel tank may not have adesired accuracy. When mission planners do not know with a high degreeof confidence how much fuel is left in the satellite's fuel tank, thesatellite may be retired prematurely, which may be wasteful of theremaining fuel and of remaining useful life of the satellite.

SUMMARY

Systems and methods to determine an amount of a fluid remaining in atank are disclosed. The disclosed systems and methods are accurate anddo not require that new hardware be added to the tank.

In a particular embodiment, a tank to contain a fluid includes a tankwall having an inner surface. A vane is disposed within the tank. Thevane is configured to facilitate extraction of fluid from the tank. Thevane may include an electrically conductive material. The tank alsoincludes a first connector to electrically couple at least one portionof the tank wall to a capacitance sensing device and a second connectorto electrically couple at least one portion of the vane to thecapacitance sensing device. When the capacitance sensing device iscoupled to the first connector and to the second connector, thecapacitance sensing device can measure a capacitance between the atleast one portion of the vane and the at least one portion of the tankwall. When the fluid stored in the tank is electrically conductive, anelectrically insulating layer may be provided on a surface of a hardwarecomponent inside the tank to prevent electrical shorting between thevane and the tank wall.

In another particular embodiment, a mobile platform (such as asatellite) includes a tank coupled to the mobile platform. A vane isdisposed within the tank and configured to facilitate extraction ofliquid from the tank. The vane includes an electrically conductivematerial. An electrically insulating layer may be disposed between thevane and an inner surface of a wall of the tank. The satellite alsoincludes a capacitance sensing device coupled to the tank and to thevane. The capacitance sensing device is configured to measure acapacitance between the vane and the inner surface of the wall of thetank.

In another particular embodiment, a method includes receivinginformation indicative of a capacitance between an inner surface of awall of a tank and a vane disposed within the tank. The vane includes anelectrically conductive material. A capillary channel between the vaneand the inner surface of the wall may facilitate extraction of liquidfrom the tank. The method also includes determining an amount of theliquid that is in the tank based on the capacitance.

The features, functions, and advantages that have been described can beachieved independently in various embodiments or may be combined in yetother embodiments, further details of which are disclosed with referenceto the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a particular embodiment of a tank;

FIG. 2 is a graph illustrating a change in capacitance relative to achange in a fluid level in a tank according to a particular embodiment;

FIG. 3 is a diagram illustrating a particular embodiment of a mobileplatform including a tank;

FIG. 4 is flow chart of a particular embodiment of a method ofdetermining a fluid level in a tank; and

FIG. 5 is a block diagram of a general purpose computer system operableto perform computer-implemented methods or process computer-executableinstructions according to a particular embodiment.

DETAILED DESCRIPTION

Generally, a satellite may be removed from its orbital position to aparking location or de-orbited before all fuel onboard the satellite isused. This allows the satellite's orbital location to be used by afuture satellite and prevents accumulation of debris in useful orbits.Accordingly, the satellite may be prematurely retired by removing thesatellite from its orbit when uncertainty regarding the amount of fuelleft onboard the satellite becomes too large. Certain methods ofestimating an amount of fuel left in a satellite fuel tank may lead toconsiderable uncertainty. For example, a “book-keeping” method may relyon users to accurately calculate and record each fuel use (e.g., whenthrusters were burned, a length of time the thrusters were burned, andthrust and flow rates the thrusters produced). With this data, the userscan estimate fuel used and subtract the used fuel from a starting fuelamount that was originally put into the tank. Using the book keepingmethod, measurement errors regarding fuels usage or errors in loggingfuel usage may lead to early retirement of the satellite (e.g., while itstill has sufficient fuel onboard to remain in orbit).

In another example, a gas law estimation technique may use the Ideal GasLaw (or another gas law) to estimate fuel volume in the tank. Toillustrate, temperature and pressure readings may be taken from the fueltank. The pressure readings may be readings of pressure due to apressurizing gas (e.g., Helium) in the tank. The fuel volume remainingin the tank may be estimated by first using a gas law equation toestimate a volume of pressurizing gas that is in the tank. The volume ofthe pressurizing gas is subtracted from the total volume of the tank toestimate fuel volume remaining.

In yet another example, a thermal propellant gauging method may be used.The thermal propellant gauging method is based upon the concept ofmeasuring a thermal capacitance of the tank (including the fuel andpressurizing gas). The tank and its contents are subjected to heating.An observed temperature rise due to the heating is compared to simulatedtank thermal model results. This model is based on a thermal model of afuel tank that includes a first node for a gas portion of contents ofthe propellant tank and a second node for a liquid portion of thecontents of the propellant tank. The amount of fuel left in the tank isestimated based on the comparison to the simulated tank thermal modelresults.

Errors that can occur using the book-keeping, gas law and thermalpropellant gauging methods may lead to leaving the satellite in a usefulorbit after the satellite does not have sufficient fuel remaining tosafely park or de-orbit the satellite. To avoid this situation, safetymargins are generally built into procedures so that sufficient fuel isavailable to park the satellite for retirement. However, these safetymargins increase the likelihood of the satellite being retired tooearly.

Further, the gas law and thermal response methods described aboverequire addition of equipment to the tank. For example, temperature andpressure sensors may be needed to calculate the amount of fuel in thetank using the gas law method. To use the thermal response method,temperature sensors and heaters may be used. This added equipment canincrease weight of the satellite, which increases cost. Additionally,adding this equipment to the tank can increase complexity and cost ofthe tank.

FIG. 1 is a diagram illustrating a particular embodiment of a tank,designated 100. The tank 100 may be adapted to contain a fluid, such asa liquid fuel. For example, the tank 100 may be configured to store aliquid fuel used by a space-based vehicle, an airborne vehicle, aland-based vehicle or a water-based vehicle.

The tank 100 may include a tank wall 102. The tank wall 102 may beformed of various materials depending upon, among other things, theparticular application and operating environment of the tank 100. Forexample, the tank wall 102 may be constructed of a metal, a polymer, acomposite material, a ceramic material, another material, or acombination of materials. In a particular embodiment, at least a portionof an inner surface 104 of the tank wall 102 includes an electricallyconductive material, such as a metal.

One or more vanes 106 are disposed within the tank 100. When the tank100 is a fuel tank, the vanes 106 may be plates that are used for fuelmanagement. For example, the vanes 106 may use properties of fluidphysics to direct fuel to the tank wall 102 to be used by a spacecraftthat is in orbit. At least one of the vanes 106 is configured tofacilitate extraction of fluid from the tank 100 (e.g., via an outlet110 of the tank 100). The vanes 106 and the inner surface 104 of thetank wall 102 may form boundaries of a capillary channel 108. Thecapillary channel 108 may facilitate movement of the fluid toward theoutlet 110 of the tank 100. For example, the fluid may be moved alongthe capillary channel 108 by capillary action 126.

To illustrate, in a microgravity environment, such as onboard anorbiting space-based vehicle, fluid feed mechanisms that rely on gravityeither to extract fluid from the tank 100 or to cause the fluid tosettle near the outlet 110 may be ineffectual. However, the capillarychannel 108 may utilize the capillary action 126 to urge the fluidtoward the outlet 110 and may thus facilitate extraction of the fluidfrom the tank 100 even in a microgravity environment.

The capillary action 126 is driven by material properties of the fluidand materials around the fluid, such as a material of the inner surface104 of the tank wall 102, a material of the vanes 106 (or a coating onthe vanes 106), and possibly other materials that may be in contact withthe fluid, such as a pressurizing fluid. Accordingly, the fluid, thetank wall 102 (in particular the inner surface 104 of the tank wall102), and the vanes 106 may be formed of materials that provideappropriate surface tension and contact angle to facilitate driving thefluid toward the outlet 110 of the tank 100. Additionally, the materialsused for the fluid, the tank wall 102 (in particular the inner surface104 of the tank wall 102), and the vanes 106 may be chemically stable(e.g., substantially non-reactive with one another).

Further, a geometric configuration of the inner surface 104 of the tankwall 102 and the vanes 106 may be designed to urge the fluid toward theoutlet 110. To illustrate, spacing between the inner surface 104 of thetank wall 102 and the vanes 106 may be selected to provide a capillaryaction force in a direction toward the outlet 110.

In a particular embodiment, an insulating layer 122 may be provided onthe inner surface 104 of the tank wall 102, the vanes 106, or both. Forexample, the insulating layer 122 may include a polymer coating. Theinsulating layer 122 may electrically insulate the tank wall 102 fromthe vanes 106. In some embodiments, the vanes 106 may be structurallyattached to the tank wall 102, to other components of the tank 100(e.g., to attachment points at an inlet 114 and the outlet 110 of thetank 100), or a combination thereof. In some such embodiments, theinsulating layer 122 may be provided only at these structural attachmentpoints. In these embodiments, physical separation of the tank wall 102and the vanes (e.g., via the capillary channel 108) may provideelectrical isolation of the tank wall 102 and vanes 106 in areas otherthan the structural attachment points. For example, when the fluid to bestored in the tank 100 is not electrically conductive, providing theinsulating layer 122 only at the structural attachment points may besufficient. In other embodiments, such as when the fluid to be stored inthe tank 100 is conductive, the insulating layer 122 may substantiallycover the inner surface 104 of the tank wall 102, the vanes 106, orboth. In these embodiments, a material of the insulating layer 122 maybe selected to have desired material properties, such as a contact anglewith the fuel that facilities the capillary action 126, chemicalstability, dielectric constant, and so forth.

In some embodiments, the material used for the insulating layer 122 ishighly corrosion-resistant (e.g., able to withstand a corrosiveenvironment of a satellite fuel tank for 15-20 years). In a particularembodiment, when the fluid to be contained in the tank is an electricalinsulator, no insulating layer 122 may be used. Alternately, theinsulating layer 122 may be present even when the liquid is an insulatorsince the insulating layer 122 may also provide other desired materialproperties, such as corrosion resistance. In a particular embodiment,suitable materials for the insulating layer 122 may include, but are notlimited to: AF-E-332 (a Hydrazine resistant coating) and Teflon.

In a particular embodiment, the vanes 106 are configured to inhibitsloshing of the fluid in the tank 100. In some embodiments, the vanes106 provide structural support for the tank wall 102 to stabilizedimensional characteristics of the tank 100.

Since the inner surface 104 of the tank wall 102 and the vanes 106 maybe electrically insulated from one another (e.g., by the capillarychannel 108 and the fluid, by the insulating layer 122, or by acombination thereof), the tank wall 102 and the vanes 106 may act as acapacitor. In a particular embodiment, capacitance of the capacitorformed by the inner surface 104 of the tank wall 102 and the vanes 106varies depending on how much fluid is in the tank 100. For example, thecapacitance between the inner surface 104 of the tank wall 102 and thevanes 106 may decrease as an amount of fluid in the tank 100 decreases.To illustrate, when the tank 100 is full of the fluid, the inner surface104 of the tank wall 102 and the vanes 106 may have a first capacitance.When none of the fluid remains in the tank (although another fluid, suchas a pressurizing gas, may remain in the tank 100) the inner surface 104of the tank wall 102 and the vanes 106 may have a second capacitancethat is different than the first capacitance. Further, the capacitancebetween the inner surface 104 of the tank wall 102 and the vanes 106 maychange according to a determinable function between the firstcapacitance and the second capacitance as the amount of fluid in thetank 100 is changed. For example, the function may be determined throughexperimentation to identify an empirical relationship between thecapacitance and the amount of fluid in the tank 100. After the empiricalrelationship has been identified, the amount of fluid in the tank 100may be calculated using the empirical relationship based on a measuredcapacitance between the inner surface 104 of the tank wall 102 and thevanes 106.

To illustrate, a capacitance sensing device 120 may be coupled to thetank wall 102 and to one or more of the vanes 106 (e.g., via one or moreelectrical connectors 116 and 118). Although an electrical connector 116is shown in FIG. 1 as inside the tank 100, in certain embodiments, theelectrical connector 116 may be external to the tank 100 and may beelectrically coupled to a portion of one or more of the vanes 106. Thecapacitance sensing device 120 may measure the capacitance between atleast one portion of the vane 106 and at least one portion of the tankwall 102. Alternately or in addition, the capacitance sensing device 120may measure other information that is indicative of capacitance, such asa voltage between the tank wall 102 and the vanes 106. An amount of thefluid present in the tank 100 may be determined based on the measuredcapacitance. For example, the capacitor formed by the inner surface 104of the tank wall 102 and the vanes 106 may be modeled as a parallelplate capacitor, either considering or ignoring fringe effects. Toillustrate, modeling the capacitor as a parallel plate capacitor, alength of the capacitor may be effectively the same as the length aroundthe edge of the vane 106 or the length around the corresponding innersurface 104 of the tank wall 102. When the tank is full, a singlecapacitor may be formed that includes a liquid dielectric (i.e., thefluid stored in the tank 100) throughout its length. As the liquid leveldecreases, two capacitors may be formed, one with the liquid dielectricand one with a gaseous dielectric (e.g., from a pressurizing gas in thetank 100). The gaseous dielectric may have a lower dielectric constant(i.e., relative permittivity) than the liquid dielectric. With thephysical geometry of a capacitor held constant, the lower the dielectricconstant, the lower the measurable capacitance. Thus, as the liquidlevel decreases, the total measurable capacitance decreases.

Alternately, the relationship between the capacitance of the capacitorformed by the tank wall 102 and the vanes 106 and an amount of fluid inthe tank may be determined through testing, as shown in FIG. 2 anddescribed in further detail below. For a more accurate understanding ofthe relationship, reduced gravity experiments may be conducted inaddition to or instead of the bench top tests described with referenceto FIG. 2.

In a particular embodiment, the capillary channel 108 is a part of thecapacitor formed by the inner surface 104 of the tank wall 102 and thevanes 106. Due to material properties of the inner surface 104 of thetank wall 102, the vanes 106, the insulating layer 122, the fluid, or acombination thereof, the fluid may fill the capillary channel 108 whilethere is enough fluid in the tank 100 to fill the capillary channel 108(with the some possible exceptions, such as when the fluid is dispersedin the tank as a result of movement of the tank 100). Thus, generally,as long as there is enough fluid in the tank 100 to fill the capillarychannel 108, the capillary channel 108 will be filled with the fluid.

When there is not enough fluid in the tank 100 to fill the capillarychannel 108, the fluid in the capillary channel 108 may break up andbecome discontinuous. The discontinuity of the fluid in the capillarychannel 108 may cause a rate of change of the capacitance of thecapacitor formed by the inner surface 104 of the tank wall 102 and thevanes 106 to increase. To illustrate, when the tank 100 has a relativelylarge amount of fluid (i.e., such that a continuous film of the fluid ispresent in the capillary channel 108) a relatively small change in theamount of fluid present in the tank 100 may cause a relatively smallchange in the capacitance since a continuous film of the fluid mayremain present in the capillary channel 108 despite the change in theamount of fluid present in the tank 100. However, as the tank 100approaches empty, a relatively small change in the amount of fluidpresent in the tank 100 (e.g., from a first level 128 to a second level130) may cause a discontinuity in the film of the fluid in the capillarychannel 108, which may cause a relatively large change in thecapacitance. Accordingly, a relationship of the amount of fluid in thetank 100 to the capacitance between the tank wall 102 and the vane 106may be more sensitive as the tank 100 approaches empty, providingimproved fluid level measurement sensitivity as the tank empties.

Thus, the tank 100 may provide fluid level sensing (i.e., a measure ofthe amount of the fluid in the tank 100) without the addition of levelsensing hardware inside the tank 100. As such, elements of the tank 100that are present to enable operation of the tank 100, e.g., the tankwall 102 and the vanes 106, may be used to provide level sensing. Sinceno additional level sensing hardware is added to the tank 100, the tank100 may be smaller, lighter, or both. Additionally, the tank 100 mayprovide improved fluid level measurement sensitivity as the tank 100approaches empty, which may allow more of the fluid to be used withoutconcerns regarding maintaining a minimum amount of fluid in reserve inthe tank 100. For example, satellite operators may be expected to retaina fuel reserve that can be used at the end of a satellite's useful lifeto remove the satellite from a particular orbit. Thus, accuratemeasurement of fuel level as the satellite's fuel tank (e.g., the tank100) approaches an empty state may extend the useful life of thesatellite resulting in substantial cost savings and reduced waste.

FIG. 2 is a graph 200 illustrating a relationship between a change incapacitance and a change in fluid level in a tank according to aparticular embodiment. Experimental measurements were performed togenerate the graph 200. For the experimental measurements, an insulatedvane was placed in a portion of a metal tank (approximately a bottomhalf of a tank). A measured volume of liquid was added to the tank.Water was used as the liquid for the experimental measurements. Aportion of the tank and vane were connected to a capacitance meter and acapacitance measurement was taken with the known volume of liquid in theportion of the tank. A measured volume of the liquid was removed fromthe portion of the tank and another capacitance measurement was taken.This experimental process continued until the only liquid remaining inthe tank was in the tank drain hole. The entire experimental process wasrepeated several times.

Lines 202 of the graph 200 illustrate experimental results of threetests in which capacitance between the tank and the vane was measuredrelative to the volume of liquid in the tank. Note that a slope of thelines 202 becomes shallower as the tank is emptied. This change in slopeof the lines 202 corresponds to an increasing rate of change incapacitance per unit of liquid removed from the tank as the tank becomesempty. Stated another way, the changing slope of the lines 202 showsthat as the tank empties, the capacitance becomes more sensitive tochanges in the volume of fluid present in the tank. When the only fluidremaining in the tank was in the drain hole (in the area 204), thecapacitance ceased to change since no fluid remained between the tankand the vane. That is, removal of the liquid in the drain hole did notchange the capacitance of the tank and vane experimental apparatus.

FIG. 3 is a diagram illustrating a particular embodiment of a mobileplatform, such as a satellite 300, that includes a tank 310. Thesatellite 300 includes a satellite platform 302 (e.g., a body includingstructural components and, perhaps, a skin or covering). The satellite300 may also include a plurality of onboard systems 304, a payload or acombination thereof.

The tank 310 may be coupled to the satellite platform 302. The tank 310may be adapted to contain a fluid used by a system onboard the satellite300. For example, the tank 310 may be a fuel tank to contain fuel usedby a propulsion system 318 of the satellite 300. In another example, thetank 310 may contain another fluid that is provided to other systems 326of the satellite 300 via a fluid delivery system (not shown). The othersystems 326 may include, for example, communications systems, imagingsystems, navigation systems, and so forth.

A vane 312 may be disposed within the tank 310. The vane 312 may beconfigured to inhibit sloshing of the liquid in the tank 310. The vane312 may also be configured to facilitate extraction of liquid from thetank 310.

In a particular embodiment, the vane 312 includes an electricallyconductive material and at least a portion of an inner surface of thewall of the tank 310 includes an electrically conductive material. Anelectrically insulating layer 314 may be disposed between the vane 312and the inner surface of a wall of the tank 310. Thus, the vane 312 andthe inner surface of the wall of the tank 310 may form a capacitor. Thevane 312 and the wall of tank 310 may also form boundaries of acapillary channel that facilitate movement of the liquid in the tank toan outlet of the tank 310. For example, the capillary channel mayfacilitate movement of the liquid to an outlet of the tank 310 that iscoupled to a fuel line 316 coupled to the propulsion system 318.

A capacitance sensing device 320 may be coupled to the tank 310 and tothe vane 312. The capacitance sensing device 320 may be configured tomeasure a capacitance between the vane 312 and the inner surface of thewall of the tank 310. The capacitance between the vane 312 and the innersurface of the wall of the tank 310 may change as an amount of theliquid in the tank 310 changes. For example, the capacitance may changewhen a level of the liquid in the tank 310 is insufficient to fill thecapillary channel.

The satellite 300 may also include a controller 322 coupled to thecapacitance sensing device 320. The controller 322 may be configured todetermine a liquid level in the tank 310 based on the capacitancemeasured by the capacitance sensing device 320. The satellite 300 mayalso include a transmitter 324 to send information to a ground-basedreceiver. For example, the transmitter 324 may send the informationindicative of the measured capacitance via a satellite downlink signal330 to a ground station 334. In another example, the transmitter 324 maysend information indicative of an amount of fluid in the tank 310 to theground station 334 via the satellite downlink signal 330.

Thus, an amount of fuel that remains in the tank 310 can be determinedwith confidence. Knowing the amount of fuel that remains in the tank 310may extend the operational life of the satellite 300 since an operatorof the satellite 300 can be confident that sufficient fuel remains toremove the satellite 300 from orbit. Additionally, the tank 310 providesfluid level sensing (i.e., a measure of the amount of the fluid in thetank) without the addition of level sensing hardware inside the tank310. Rather, elements of the tank 310 that are present to enableoperation of the tank 310, e.g., the tank wall and the vane 312, mayalso be used to provide fluid level sensing. Since no additional levelsensing hardware is added within the tank 310, the tank 310 may besmaller, lighter, or both, which may reduce the cost of launching thesatellite 300 relative to larger more complex tanks.

FIG. 4 is flow chart of a particular embodiment of acomputer-implemented method of determining a fluid level in a tank. Themethod of FIG. 4 may be implemented automatically, without humanintervention. For example, the method may be performed by a computingdevice, such as a processor executing instructions or an applicationspecific integrated circuit, as shown FIG. 5 and described in moredetail below.

The method includes, at 402, receiving information indicative of acapacitance between an inner surface of a wall of a tank and a vanedisposed within the tank. In an illustrative embodiment, the informationindicative of the capacitance may include capacitance data transmittedfrom a satellite that houses that tank. For example, the informationindicative of the capacitance may be received at a ground station, suchas the ground station 334 of FIG. 3, via a downlink signal from asatellite, such as the satellite 300. In another illustrativeembodiment, the information indicative of the capacitance may include acapacitance measurement that is received from a capacitance sensingdevice by a controller, such as the controller 332 onboard the satellite302 of FIG. 3, or another computing device onboard the satellite 302.The information indicative of the capacitance may include digitizedelectrical values, such as capacitance, or other measurements that canbe used to calculate the capacitance based on information about thetank, the vane, the fluid, other components of a system in which thetank is housed, or any combination thereof.

The vane may include an electrically conductive material. The innersurface of the wall of the tank may also include an electricallyconductive material. An electrically insulating layer may be disposed onthe inner surface of the wall of the tank, on the vane, or both. Thevane may facilitate extraction of liquid from the tank. For example, acapillary channel between the vane and the inner surface of the wall ofthe tank may facilitate movement of the liquid to an outlet of the tankusing capillary action.

The method may also include, at 404, determining an amount of liquidthat is in the tank based on the capacitance. For example, the amount ofliquid in the tank may be calculated or otherwise determined (e.g., viaa lookup table) based on the capacitance. To illustrate, the capacitancemay change (e.g., increase or decrease) as the amount of liquid in thetank decreases. A relationship between the capacitance and the amount ofthe liquid may be determined via testing. Sets of curves, such as thelines 202 of FIG. 2, may be prepared with different curves correspondingto different geometries of tank and vane design (i.e., spherical,rectangular, irregular, etc.), different fluids, different materials ofconstruction (e.g., different tank wall materials, different vanematerials, different insulating layer materials, etc.), otherdifferences in particular systems, or any combination thereof. In someembodiments, “look-up” tables of capacitance values corresponding tofuel levels in tanks of different systems may be generated.

In an illustrative embodiment, fuel level in a tank onboard a vehicle,such as the satellite 300 of FIG. 3, may be determined using the methoddescribed in FIG. 4. To illustrate, a computing device onboard thesatellite, such as the controller 322, may receive informationindicative of capacitance between the inner surface of the tank 310 andthe vane 312 from the capacitance sensing device 320. The controller 322may calculate the amount of liquid that is in the tank 310 based on thereceived information that is indicative of the capacitance. Informationindicating the amount of liquid in the tank may then be transmitted,e.g., by the transmitter 324, to the ground station 334. Alternately orin addition, the information indicative of the capacitance may betransmitted by the transmitter 324 to the ground station 334 via thesatellite downlink signal 330. A computing device at the ground station334 may calculate the amount of liquid that is in the tank 310 based onthe received information that indicates the measured capacitance. Thus,the tank 310 can provide fluid level sensing without the addition oflevel sensing hardware inside the tank 310.

FIG. 5 is a block diagram of a general purpose computer system 500operable to perform computer-implemented methods or to processcomputer-executable instructions to determine an amount of a fluid in atank based on a capacitance measurement. The computer system 500 may bepositioned near the tank such as onboard a satellite with the tank. Forexample, the computer system 500 may be a portion of or may be includedwithin the controller 332 of FIG. 3. Alternately or in addition, thecomputer system 500 may be located remote from the tank. For example,the computer system 500 may be located at a ground station, such as theground station 334 of FIG. 3 and may receive information indicative ofthe capacitance measurement via the satellite downlink 330 or viaanother communication signal.

In an illustrative embodiment, a computing device 510 of the computingsystem 500 may include at least one processor 520. The processor 520 maybe configured to execute instructions to implement a method ofdetermining an amount of fluid in a tank based on a capacitancemeasurement. The processor 520 may communicate with a system memory 530,one or more storage devices 540, and one or more input/output devices570, via input/output interfaces 550.

The system memory 530 may include volatile memory devices, such asrandom access memory (RAM) devices, and nonvolatile memory devices, suchas read-only memory (ROM), programmable read-only memory, and flashmemory. The system memory 530 may include an operating system 532, whichmay include a basic input/output system (BIOS) for booting the computingdevice 510 as well as a full operating system to enable the computingdevice 510 to interact with users, other programs, and other devices.The system memory 530 may also include one or more application programs534.

The processor 520 also may communicate with one or more storage devices540. The storage devices 540 may include nonvolatile storage devices,such as magnetic disks, optical disks, or flash memory devices. In analternative embodiment, the storage devices 540 may be configured tostore the operating system 532, the applications 534, the program data536, or any combination thereof. The processor 520 may communicate withthe one or more communication interfaces 560 to enable the computingdevice 510 to communicate with other computing systems 580.

The disclosed embodiments utilize information (e.g., the measuredcapacitance) that is directly related to the amount of fluid that isremaining inside of the tank, unlike other techniques that rely uponoutside factors to determine an estimate of the fluid remaining. Forexample, the gas law estimation above requires multiple measurements(e.g., temperature and pressure inside the tank) thereby having multiplesources for error, uses hardware inside the tank (e.g., temperature andpressure probes), and indirectly calculates the amount of fluidremaining in the tank by determining a volume of a pressurizing gas thatis present in the tank. In another example, the thermal propellantgauging method may be subject to error based on the placement of thetemperature sensors and heaters. Further, the thermal propellant gaugingmethod requires that temperatures sensors be present in the tank.Accuracy of the thermal propellant gauging method relies on accuratemeasurement of heat applied to the tank and to the fluid and may beinfluenced by whether the heat is applied uniformly. Further, complexmathematical simulation models may be used which may be cumbersome andmay be subject to their own errors (such as uncertainty regardingheating due to sunlight when a satellite passes into or out of eclipse).In yet another example, the book-keeping method described above uses aknown starting volume of the fluid in the tank and calculates fuelremaining indirectly by subtracting fuel used based on flow rates andtimes. Inaccuracies in the starting volume and in each measurement usedto estimate the fluid use may accumulate over multiple individual fluiduses to a large error when the fuel tank is nearly empty. Accordingly,the disclosed embodiments may enable satellites (and possibly othervehicles) to remain in service longer and at lighter weight.Additionally, the disclosed embodiments do not introduce new hardware(e.g., heating devices, pressure measurement devices, etc.) into a tanksystem. Thus, weight and complexity of the tank may be reduced.Additionally, design complexity is reduced, which may reduce overallsystem integration complications.

The illustrations of the embodiments described herein are intended toprovide a general understanding of the structure of the variousembodiments. The illustrations are not intended to serve as a completedescription of all of the elements and features of apparatus and systemsthat utilize the structures or methods described herein. Many otherembodiments may be apparent to those of skill in the art upon reviewingthe disclosure. Other embodiments may be utilized and derived from thedisclosure, such that structural and logical substitutions and changesmay be made without departing from the scope of the disclosure. Forexample, method steps may be performed in a different order than isshown in the figures or one or more method steps may be omitted.Accordingly, the disclosure and the figures are to be regarded asillustrative rather than restrictive.

Moreover, although specific embodiments have been illustrated anddescribed herein, it should be appreciated that any subsequentarrangement designed to achieve the same or similar results may besubstituted for the specific embodiments shown. This disclosure isintended to cover any and all subsequent adaptations or variations ofvarious embodiments. Combinations of the above embodiments, and otherembodiments not specifically described herein, will be apparent to thoseof skill in the art upon reviewing the description.

The Abstract of the Disclosure is submitted with the understanding thatit will not be used to interpret or limit the scope or meaning of theclaims. In addition, in the foregoing Detailed Description, variousfeatures may be grouped together or described in a single embodiment forthe purpose of streamlining the disclosure. This disclosure is not to beinterpreted as reflecting an intention that the claimed embodimentsrequire more features than are expressly recited in each claim. Rather,as the following claims reflect, the claimed subject matter may bedirected to less than all of the features of any of the disclosedembodiments.

What is claimed is:
 1. A tank comprising: a wall having an innersurface, wherein an insulating layer is disposed on a first portion ofthe inner surface; a vane that is disposed within the tank and that isconfigured to facilitate extraction of a fluid from the tank, whereinthe vane comprises an electrically conductive material; a firstconnector to electrically couple a portion of the wall to a capacitancesensing device; and a second connector to electrically couple a portionof the vane to the capacitance sensing device, wherein, when thecapacitance sensing device is coupled to the first connector and iscoupled to the second connector, the capacitance sensing device isoperable to measure a capacitance between the portion of the vane andthe portion of the wall, wherein the capacitance sensing device iscoupled to a controller, the controller operable to receive informationrepresentative of the capacitance between the portion of the vane andthe portion of the wall and to apply a function to the informationrepresentative of the capacitance between the portion of the vane andthe portion of the wall to determine an amount of the fluid remaining inthe tank, and wherein the function is associated with a relationshipbetween the amount of the fluid that is contained in the tank and thecapacitance.
 2. The tank of claim 1, wherein an insulating material isdisposed between the vane and the inner surface of the wall.
 3. The tankof claim 1, further comprising a second vane that is disposed within thetank and that is configured to facilitate extraction of the fluid fromthe tank.
 4. The tank of claim 1, wherein the inner surface of the wallcomprises an electrically conductive material.
 5. The tank of claim 1,wherein the capacitance measured by the capacitance sensing devicedecreases as a level of the fluid in the tank decreases.
 6. The tank ofclaim 1, further comprising an outlet, wherein the vane and the innersurface of the wall form boundaries of a capillary channel, thecapillary channel facilitating movement of the fluid toward the outlet.7. The tank of claim 6, wherein the capacitance measured by thecapacitance sensing device decreases in response to fluid in thecapillary channel becoming discontinuous.
 8. A system comprising: aprocessor; and a memory accessible to the processor, the memoryincluding instructions that are executable by the processor to: receiveinformation indicative of a capacitance between an inner surface of awall of a tank and a vane disposed within the tank, wherein aninsulating layer is disposed on a first portion of the inner surface;and apply a function to the information indicative of the capacitancebetween the inner surface of the wall and the vane to determine anamount of liquid that is contained in the tank, wherein the function isassociated with a relationship between the amount of liquid that iscontained in the tank and the capacitance.
 9. The system of claim 8,wherein the insulating layer is resistant from corrosion.
 10. The systemof claim 8, wherein a second insulating layer is disposed on the vane.11. The system of claim 8, wherein the inner surface of the wallcomprises an electrically conductive material.
 12. The system of claim8, wherein the capacitance decreases as the amount of the liquidcontained in the tank decreases.
 13. The system of claim 8, wherein thevane and the inner surface of the wall form boundaries of a capillarychannel, the capillary channel facilitating movement of the liquidtoward an outlet of the tank.
 14. The system of claim 8, wherein theinformation is received via a downlink signal from a satellite, whereinthe tank is onboard the satellite.
 15. The system of claim 8, furthercomprising receiving second information indicative of the amount of theliquid that is contained in the tank via a downlink signal from asatellite, wherein the tank is onboard the satellite.
 16. A mobileplatform comprising: a tank, wherein an insulating layer is disposed ona first portion of an inner surface of a wall of the tank; a vane thatis disposed within the tank and that is configured to facilitateextraction of liquid from the tank, wherein the vane comprises anelectrically conductive material; and a capacitance sensing devicecoupled to the tank and to the vane, wherein the capacitance sensingdevice is configured to measure a capacitance between the vane and theinner surface of the wall of the tank, wherein the capacitance sensingdevice is communicatively coupled to a controller, the controlleroperable to receive information representative of the capacitancebetween the vane and the inner surface of the wall and to apply afunction to the information representative of the capacitance betweenthe vane and the inner surface of the wall to determine an amount of theliquid remaining in the tank, wherein the function is associated with arelationship between the amount of the liquid that is contained in thetank and the capacitance.
 17. The mobile platform of claim 16, whereinthe vane is further configured to inhibit sloshing of the liquid in thetank.
 18. The mobile platform of claim 16, further comprising apropulsion system coupled to the tank, wherein the liquid comprises afuel that is for use by the propulsion system.
 19. The mobile platformof claim 16, wherein the measured capacitance decreases as the amount ofthe liquid contained in the tank decreases.
 20. The mobile platform ofclaim 16, wherein the vane and the wall form boundaries of a capillarychannel, the capillary channel facilitating movement of the liquidtoward an outlet of the tank, and wherein the measured capacitancechanges when a level of the liquid in the tank is insufficient to fillthe capillary channel.